During the pandemic, we talked a lot about the effects of school closures. Sometimes based on experience, sometimes on anecdotes, and only occasionally on solid research, often from other contexts such as teacher strikes. A new study by Pelin Ozluk and colleagues in California clearly belongs to that last category. However, I sincerely hope we will not need these kinds of insights again anytime soon.

The researchers used a form of natural experiment. Schools did not reopen everywhere at the same time. That variation allowed comparison between students who could return to school and those who had to stay home a little longer. That sounds simple, but the method they used is, in my view, state-of-the-art for this type of situation.

The finding is both straightforward and uncomfortable. Straightforward, because the main conclusion fits into a single sentence. Uncomfortable, because many people hoped the effect would be more negligible. Nine months after reopening, the share of children receiving a mental health diagnosis had dropped by about 1.2 percentage points. Most of that decline came from fewer diagnoses of depression and, a little later, anxiety. Total medical spending on mental health also fell by roughly ten percent. The effects were most substantial among girls, a group that is generally at higher risk for mental health problems and therefore seemed to benefit most when schools reopened.

Does this mean school closures caused all the mental health problems we saw in children and adolescents over the past years? No. There are plenty of other possible explanations. And the authors make that clear as well. The dataset consists of children from relatively affluent, insured families. Other measures changed at the same time. Not all children returned to in person learning at the same pace, even within the same district. So you see a clear pattern, but you know there will always be noise. Still, it is hard to ignore that something happens when schools reopen. Structure, social contact, access to support, and daily routines. None of these is spectacular, but together they matter.

This is relevant for future policymaking. Not because this study tells us everything, but because it provides another piece of the puzzle that helps weigh risks and benefits during a crisis. More than anything, the results remind us that school is more than a place where learning goals are ticked off. Sometimes you only see what a system carries when you take it away for a moment.